1. Field of the Invention
The present invention relates to a crystallization apparatus, a crystallization method, a method of manufacturing a thin film transistor, a thin film transistor, and a display apparatus, and in particular, to a crystallization apparatus and method that enables real-time observation and monitoring a process of how a semiconductor thin film is melted and crystallized, and a method of manufacturing a thin film transistor, a thin film transistor, and a display apparatus, which are processed using a semiconductor thin film manufactured by the crystallization apparatus and method.
2. Description of the Related Art
A crystallization technique is used to crystallize a semiconductor thin film forming a thin film transistor (TFT) used in, for example, a liquid crystal display apparatus or an organic electroluminescence display apparatus; the crystallization technique comprises melting and crystallizing a semiconductor thin film using an energy beam, for example, a laser light with a high energy such as short pulse laser light. The inventor has been developing a liquid crystal display apparatus for a large screen. For example, a switching device for a pixel section in a liquid crystal display apparatus is composed of a thin film transistor. The switching device is formed in a silicon thin film crystallized so as to have a large grain size because it must be able to operate at high speed. The crystallized silicon thin film is formed by, for example, a laser crystallization technique to crystallize an amorphous silicon thin film formed on a support substrate such as a large glass substrate.
Among such crystallization techniques, a technique has been gathering much attention which carries out crystallization by applying a phase-modulated excimer laser light (i.e. Phase Modulated Excimer Laser Annealing (PMELA)). The PMELA technique is a method to melt and crystallize the silicon thin film, for example, an amorphous silicon film or a polycrystal silicon thin film, by irradiating a semiconductor thin film with a pulse excimer laser light having its phase modulated by a phase shifter and having a predetermined light intensity distribution. A crystallized silicon thin film having a large grain size can be obtained by properly controlling the crystallization process. The currently developed PMELA technique forms a high-quality crystallized silicon thin film having relatively uniform crystal grains of grain size several to about 10 μm. The crystallized silicon thin film is formed by melting and crystallizing a preset place within an area of several millimeters square in a single excimer laser light irradiating operation. The details are described in, for example, “Amplitude and Phase Modulated Excimer-Laser Melt-Regrowth Method of Silicon Thin-Films—A New Growth Method of 2-D Position-Controlled Large-Grains—” published by Kohki Inoue, Mitsuru Nakata, and Masakiyo Matsumura in Journal of the Institute of Electronics, Information and Communication Engineers, Vol. J85-C, No. 8, pp. 624–629, 2002.
Presently, in a silicon thin film having a large crystal grain size of at least several μm, one or more thin film transistors can be formed in one crystal grain. A liquid crystal display apparatus composed of the thin film transistors can playback a uniform color image over the entire surface of a large screen and perform high-speed switching. The manufacture of semiconductor thin films having such characteristics as well as crystal grains of a large grain size must be reliable, and the quality of these semiconductor thin films must be properly managed.
With the current PMELA technology, an available excimer laser light power varies by only about 5 to 10%. However, compared to the stability of the excimer laser light, a process margin for forming crystallized silicon thin films of a desired quality is very small, for example. Thus, industrializing this technology requires the process margin to be increased in order to further improve and stabilize the quality of crystallized silicon thin films. To achieve this, it has been desired to observe or monitor a changing crystallization process, how a silicon thin film is melted and crystallized in a very small area, at a high spatial resolution of several μm and a high temporal resolution on the order of nanoseconds, in real time or immediately after irradiation with a laser light.
As such an in situ observation, examples of experiments are reported by M. Hatano, S. Moon, M. Lee, K. Suzuki, and C. Grigoropoulos in J. Applied Physics, Vol. 87, No. 1, pp. 36–43, 2000, “Excimer laser-induced temperature field in melting and resolidification of silicon thin films”. In the experiments, the thermal properties of silicon thin films were observed which were melted and crystallized using an excimer laser crystallization (ELA) technology and without using phase modulation. In this report, the thermal property of melted and crystallized silicon thin films was measured at a high temporal resolution on the order of nanoseconds (hereinafter referred to as nsec). Specifically, a helium-neon (He—Ne) laser light (wavelength: 633 and 1,520 nm), as a probe light for observation, is irradiated to melting and crystallizing area from obliquely above. Reflected and transmitted light beams from the melting and crystallizing area are detected by an indium-gallium-arsenic photo detector and a silicon pn photo diode to measure the thermal property of the silicon thin films.
In the PMELA, a silicon thin film is melted and then crystallized by being irradiated with a crystallizing laser light for several dozen to 100 nsec. The time required from the melting to the end of the crystallization is several 100 nsec. Furthermore, the crystallized area to be observed or monitored is very small and has a size of about several tens μm square. However, the method of M. Hatano et al. cannot determine what part of an amorphous silicon thin film has been melted, that is, the melted area in the amorphous silicon thin film. Naturally enough, temporal changes in the melted area cannot be measured either. In the prior art, if silicon thin film transistors manufactured on such unreliably evaluated silicon thin films are used as switching devices in a liquid crystal display apparatus, the switching devices may cause a failure in an electrical property.
Moreover, the method of M. Hatano et al. cannot measure positional information that is important in crystal growth (lateral growth), that is, rapid changes of at most 1 nsec in an area of about 1 μm square and changes in faint light. To realize a high-performance display by reducing the size of transistors while increasing the integration density, it is important in process development, in production, and in quality management to monitor what position of an amorphous silicon thin film is crystallized and how the crystallization progresses.
Therefore, the method of M. Hatano et al. provides a high temporal resolution but is not applicable to observation systems that simultaneously meet both a high spatial resolution of at most several μm and a high temporal resolution.
Further, in the laser crystallization, the lateral growth of an amorphous silicon thin film is estimated to progress at a speed of 7 m/sec. The currently reported crystal grain size is up to about several μm. Accordingly, to monitor the lateral growth during crystallization in real time, it is preferable to make measurements using a temporal resolution equal to a time (10 nsec or shorter, with a spatial resolution of sub μm) at least one order shorter than the time required for crystal growth:10−6 m/(7 m/sec)≈10−7 sec=100 nsec.
Moreover, time for phase transitions (solid-liquid-solid) is about 10 nsec according to data from a method of irradiating a crystallizing area with an observing illumination light or monitor light and measuring changes in the reflection from the crystallizing area. A resolution of one-tenth of 10 nsec, that is, 1 nsec is required to monitor the lateral growth during crystallization in real time. Thus, there are problems to observe or monitor the lateral growth based on the method of laser crystallization, for example, enabling to make measurements at a temporal resolution equal to a very short time of at most 1 nsec, enabling to make measurements at a high spatial resolution equal to a very small area of at most 1 μm, and enabling to measure an image with a very small quantity of light. Compared to the order of the time (seconds) and distance (m) to be measured, the quantity of light to be measured is much smaller, that is, about 10−9×10−6. Consequently, there is a problem in the quantity of light to be observed or monitored.
The inventor has been developing a laser crystallization apparatus provided with an observation system, that is, an optical system that enables observations at a high spatial resolution of several μm and a high temporal resolution on the order of nanoseconds in real time or during or immediately after laser melting. To incorporate the observation system into the laser crystallization apparatus, it is desirable to use an optical system that simultaneously corrects aberrations in a crystallizing excimer laser light (ultraviolet light region) and an observing illumination light (visible light region).
To achieve the above objects and requirements, the problems described below must further be solved.
From a view of production efficiency, it is a prerequisite that lenses actually used in a PMELA apparatus can provide a high light intensity and a high duty and expose a large area. Specifically, the laser light intensity is preferably approximately 1 J/cm2 on a substrate to be crystallized. Thus, in contrast to an exposure apparatus for a large integrated circuit using a similar excimer laser light, i.e. an aligner or a stepper, the PMELA apparatus uses the laser light without limiting its large spectral width (0.5 nm). Further, the excimer laser light used is, for example, krypton fluoride (KrF) excimer laser or xenon chloride (XeCl) excimer laser and has a wavelength of 248 or 308 nm, respectively. In view of these wavelengths of the laser light, available lens materials are limited; it is preferable to use synthetic quartz for UV grade or calcium fluoride (CaF2). Furthermore, a configuration with pasted lenses such as microscope lenses for visible light is not preferable in terms of heat resistance. Accordingly, the degree of freedom is limited in designing the lenses.
Moreover, in a crystallization process using a phase modulating element, for example, a phase shifter, a substrate on which crystallization process is performed is irradiated with a laser light having a predetermined light intensity distribution. Specifically, for example, a mask pattern in the phase shifter is transferred onto the substrate at a high resolution of about several μm so that the mask pattern has a reduced or unchanged size on the substrate. Thus, the lens (lens group) used in the PMELA apparatus must undergo corrections of color aberrations, strain aberrations, or the like in the ultraviolet light region. If this single optical system is used for both excimer laser light and visible light for microscopic observations, aberrations must be simultaneously corrected in the two wavelength regions, that is, the ultraviolet light region and the visible light region. This is very difficult to achieve. For example, even if color aberrations can be successfully corrected, the number of lenses must be increased. Then, the quantity of light absorbed by the lenses increase to reduce the light intensity of the laser light reaching the substrate. This is contrary to the requirement for an increased light intensity.
Moreover, in a crystallizing optical system adapted for an excimer laser light exhibiting the previously described performance, a transmitted visible light disadvantageously has a reduced resolution. Specifically, the resolution is proportional to the wavelength of the light. Accordingly, in the case of a visible light (wavelength: 480 to 600 nm) which has a wavelength twice as large as that of an excimer laser light (wavelength: 248 or 308 nm), when the excimer laser light has a resolution of 2 μm, the visible light has a reduced resolution of about 4 μm, indicating that the minimum area that can resolved by the visible light is double that can be resolved by the excimer laser light. As a result, it is impossible to obtain a resolution of at most 1 μm, which is required to observe or monitor a crystallized area of several μm.
A temporal resolution equal to a very short time (nanoseconds) is required to observe or monitor in real time how a semiconductor thin film formed on a substrate is melted and crystallized. It is thus necessary to use a high-luminance observing illumination light source or monitor light source adapted for short-time observations. When a visible light used as an observing illumination or monitor light is applied through a large number of optical lenses for excimer laser, disadvantageously not only the quantity of light is reduced but the inherent imaging performance of the ultraviolet light is also degraded.
That is, an optical system meeting such requirements must be able to be stably used at at least two different wavelengths of an excimer laser light (wavelength: for example, 248 nm) having a high light intensity (for example, at least 1 J/cm2 on the substrate), a large irradiation area (for example, at least 5×5 mm2), and a high duty (for example, a laser operation frequency of at least 100 Hz) and of an observing illumination or monitor light, for example, a visible light (wavelength: for example, 480 to 650 nm).
In the present invention, it is performed that crystallization of a semiconductor thin film by irradiating an energy beam having a predetermined intensity distribution, for example, a pulse excimer laser light having a predetermined light intensity distribution by phase modulation. The irradiated semiconductor thin film is melted then crystallized laterally controlled by energy distribution of the energy beam. The present invention makes it possible to observe or monitor how the semiconductor thin film is melted and crystallized, at a high spatial resolution of several μm and a high temporal resolution on the order of nanoseconds, in real time or immediately after the pulse laser beam irradiation. By, for example, controlling a crystallization process on the basis of the results of the observations or monitoring, the crystallization process is stabilized. It is therefore possible to provide a crystallization apparatus and method that enables a high-quality semiconductor thin film to be efficiently crystallized, a thin film transistor, a method of manufacturing a thin film transistor, and a display apparatus using the thin film transistor, which are manufactured on the semiconductor thin film using the crystallization apparatus and method.